Chlorinated solvent movement in River Terrace Gravels Debbie - - PowerPoint PPT Presentation

Chlorinated solvent movement in River Terrace Gravels

Debbie Wilkinson

National Environment Programme Restoring Sustainable Abstraction Investigation & Groundwater Catchment Management Lead

May 2019

• Background to Tonbridge

Ø Location Ø Geology Ø Chlorinated Solvents

• Monitoring Network
• Conceptualisation (simple)
• Chlorinated solvent movement

Ø Seasonal Variations / Water Levels Ø Abstraction Ø Degradation (natural attenuation)

• Options Appraisal (former Amec Foster Wheeler)
• Conclusions

2

Agenda

Above: Tonbridge Water Treatment Works

Site details:

• Tonbridge WTW is located in Tonbridge,

Kent.

• Adjacent to the River Medway.
• The catchment is semi-urban with the

town of Tonbridge to the east and rural land to the west.

• South East Water is licenced to abstract

up to 4Ml/d from the Gravel Aquifer.

3

Background to Tonbridge - Location

Above: Location and Source Protection Zone (SPZ) Map for Tonbridge Water Treatment Works. Red = SPZ 1; Green = SPZ 2; Blue = SPZ 3

4

Background to Tonbridge - Geology

Geology Description Aquifer

Drift - Alluvium Low permeability silty clay (up to 6m thick) Non-aquifer Drift - River Terrace Gravels Brick earth gravels (up to 4m thick) Secondary A aquifer Solid - Tunbridge Wells Sands Fine sands, sandstone and clay (up to 12m thick) Secondary A aquifer Solid - Wadhurst Clay Clay and shale with subordinate sandstone, ironstone and limestone (up to 60m thick) Non-aquifer (aquitard) Solid - Ashdown Beds Interbedded sandstone and limestone (70m plus thick) Secondary A aquifer

Above: Cross-section from Halcrow (2006).

Comprises of five shallow radial wells that abstract water from the River Terrace Gravel aquifer.

5

Background to Tonbridge - Geology

Above and Left: Drilling of new gravel Ranney (radial) wells at Tonbridge in 2015.

• Since mid 2000’s, chlorinated solvents have been detected in groundwater including Tetrachloroethene (PCE), Trichloroethene

(TCE), and lower concentrations of 1,2-Dichloroethane (1,2 DCA). Ø Uses of chlorinated solvents include: dry cleaning, degreaser for metals, paint striping, printer inks etc…

6

Background to Tonbridge - Chlorinated Solvents

• Highest concentrations of

chlorinated solvents are found in Gravel Well 1, which is located closest to the town of Tonbridge.

• Water abstracted from Gravel

Well 1 is required to be blended with other sources to meet the Drinking Water Standard (10 µg/l).

• Environment Agency

drilled several observation boreholes in 2011-12 in the urban area of Tonbridge (shown in red) to investigate the source

• f chlorinated solvents.
• GS1 to GS5 are South

East Water’s production Gravel Wells.

• Blue hatched area is the

capture zone for Gravel Well 1 (GS1) (under pumped conditions).

7

Chlorinated Solvents – Monitoring Network

Groundwater flow direction under non- pumped conditions

8

Conceptualisation

Chlorinated solvent concentrations increase each year in the autumn months and decrease following winter recharge to the River Terrace Gravel aquifer, however, there is a notable lag effect.

9

• 1. Chlorinated Solvents – Seasonal Variations / Water Levels

Gravel Well 1 (GS1)

The abstraction regime affects chlorinated solvent concentrations at Gravel Well 1

• as abstraction rate increases, after a lag period, chlorinated solvent concentrations also increase
• as abstraction rate ceases, after a lag period, chlorinated solvent concentrations decrease
• when abstraction rates are stable chlorinated solvent concentrations are stable

Observation Boreholes

An increase in chlorinated solvent concentration at observation boreholes under pumping and/or non-pumping conditions may help indicate the likely source of the chlorinated solvents;

• if under pumping conditions chlorinated solvents increase, then the likely source is up-gradient
• if under pumping conditions chlorinated solvents decrease, then the likely source is down-gradient
• if concentrations remain stable under all conditions it is likely that they do not lie on a flow path from the source

Uncertainties include:

• a time lag
• the natural flow direction under non-pumped conditions might not be 180 degrees (west to east)

10

• 2. Chlorinated Solvents – Abstraction

11

Chlorinated Solvents – Abstraction (Under Pumped Scenario)

12

Chlorinated Solvents – Abstraction (Under Non-Pumped Scenario)

Under favourable conditions chlorinated solvents will undergo degradation (natural attenuation). Favourable conditions are:

• reducing conditions
• where there is organic matter, or;
• ther electron donors

Evidence that degradation is occurring is through the presence of daughter

• products. In the case of PCE daughter products include:
• TCE
• 1,2 DCE
• Cis 1,2 DCE
• Vinyl Chloride

Monitoring has shown some increase in daughter products (e.g. cis 1,2 DCE) in the more contaminated observation boreholes (e.g. A1), but not at significant ratios. The persistence of chlorinated solvents in the River Terrace Gravel aquifer suggests unfavourable conditions for wider degradation to occur (possibly due to the lack of electron donors).

13

• 3. Chlorinated Solvents - Degradation

A number of remediation techniques were appraised over the suitability to reduce chlorinated solvent concentrations in the River Terrace Gravels and at the Tonbridge Gravel Well abstractions. Remediation techniques appraised included:

14

Chlorinated Solvents – Options Appraisal

Technique Description

Containment hydraulic barriers:

Abstraction of groundwater is used to prevent the movement of contaminated groundwater.

Containment in-ground barriers:

A low permeability barrier is installed in a trench to prevent the moment of contaminated groundwater.

Monitored Natural Attenuation:

The natural degradation process of chlorinated solvents without intervention.

Enhanced anaerobic biodegradation:

An organic medium is added to the aquifer to produce reducing conditions for chlorinated solvents to degrade. De- halogenating bacteria may also be required for the process to work effectively.

Phytoremediation:

Involves the uptake of chlorinated solvents via plant roots.

Chemical oxidation:

Oxidants are added to the aquifer / groundwater to oxidise the contaminants. Oxidants could include ozone, hydrogen peroxide, calcium peroxide, sodium persulfate, and sodium / potassium permanganate.

Chemical reduction:

A reductant, e.g. Zero Valent Iron is added to the aquifer. This reacts with the chlorinated solvents to break them down to less toxic daughter products.

Dual phase soil vapour extraction:

Uses a vacuum system to remove contaminated groundwater from the subsurface.

Air sparging / bio sparging:

Involves injecting air into the contaminated aquifer.

Permeable reactive barrier:

In-ground barrier that contains reactive material through which groundwater can flow through. The contaminants are broken down via the reactive material.

Pump and treat:

The contaminated plume is captured by the abstraction wells, pumped to the surface and treated.

Thermal methods:

Involves increasing the groundwater temperature to aid and accelerate chemical reactions.

In situ flushing:

Involves flooding a zone of contaminated land to remove contaminant from the soil.

Additional treatment:

Additional treatment at Tonbridge Water Treatment Works.

Do nothing:

Do nothing.

The preferred option was In Situ Chemical Reduction (ISCR) using Zero Valent Iron (ZVI) in a permeable reactive barrier due to:

• groundwater conditions in the River Terrace Gravels already appear to be mildly reducing with some naturally occurring

background iron and manganese concentrations

• technical effectiveness and durability, compared with the second preferred option of Enhanced Anaerobic Biodegradation.

15

Chlorinated Solvents – Preferred Option

Above: Example of In Situ Chemical Reduction (ISCR)

16

Conclusions

Chlorinated solvent fluctuations and movement in the aquifer is complex, due to: Ø seasonal changes and groundwater recharge, and; Ø the effects of an intermittent abstraction regime on groundwater flow causing dispersion and dilution of the chlorinated solvents. Likely source of PCE is east of observation borehole A1 and west of observation boreholes C1 and EN04. No free phase of chlorinated solvents have been found. Degradation (natural attenuation) has been shown to be ineffective, especially over reasonable timescales. The options appraisal identified that In Situ Chemical Reduction (ISCR), using Zero Valent Iron (ZVI) was the preferred option, particularly due to technical effectiveness and durability, but also that the aquifer is naturally mildly reducing.

Next Steps

• Continue monitoring and refine the hydrogeological conceptualisation.
• Undertake stakeholder engagement to promote best practice of the storage and handling of chlorinated solvents.
• Feasibility study for ISCR using ZVI, leading to pilot study, if successful…

Questions

• Chlorinated solvent movement in River Terrace Gravels
• D. Wilkinson
• South East Water
• The secondary River Terrace Gravel aquifer at Tonbridge, Kent, is designated as a Safeguard Zone for chlorinated solvents. Water abstracted from

River Terrace Gravel aquifer ranney wells, for public supply, are affected by chlorinated solvent concentrations. There is a requirement to blend raw water sources at the treatment works to meet drinking water standards.

• Under the Water Industry National Environment Programme (WINEP), South East Water has a statutory obligation to investigate the deterioration of

groundwater quality. Since 2000, South East Water have regularly detected Tetrachloroethene (PCE), Trichloroethene (TCE), and 1,2-Dichloroethane (1,2 DCA), with PCE being the main reason for failure. Several investigations (alongside the Environment Agency, local authority and consultants) have assessed groundwater flows, characterised ground conditions, monitored variations in chlorinated solvents (spatially and temporally), developed conceptual models and identified likely point sources. More recently planning applications have been interrogated to provide further knowledge on sources and a remediation option appraisal has been undertaken.

• Chlorinated solvent movement in the aquifer is complex, due to the effects of an intermittent pumping regime on groundwater flow directions causing

dispersion and dilution of the chlorinated solvents. Under favourable conditions, PCE will undergo degredation (natural attenuation) producing daughter products such as 1,2 DCA, cis-1,2 DCE and also TCE. Generally degradation occurs under reducing conditions where organic matter or other electron donors are present in the aquifer. Monitoring data has shown some increase in daughter products overtime in the more contaminated observation boreholes, however, degradation in the wider aquifer is slow, possibly limited by the lack of electron donors.

• Remediation options have been appraised using a technical evaluation criteria assessment. Results favoured In Situ Chemical Reduction (ISCR), partly

due to aquifer conditions already being marginally reducing.

18